Malaria is one of the most common infectious diseases and an enormous public-health problem. Determining the infectious species responsible for a malaria infection helps determine the course of treatment for the patient. The four common species of human malaria parasites, i.e., Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae and Plasmodium ovale, that are transmitted between humans by anopheline mosquito vectors have an ancient zoonotic origin. Zoonotic malaria is currently a relatively neglected aspect of human malaria. Alterations in patterns of human settlements in the vicinity of forests, and deforestation, are increasing human proximity to primate malaria hosts in the tropics causing zoonotic malaria to become an emerging human health concern. Primate malaria parasites that are naturally transmitted by anopheline mosquito vectors to humans at the present time include the catarrhine monkey parasites Plasmodium knowlesi and Plasmodium cynomolgi in Southeast Asia, the platyrrhine monkey parasites Plasmodium brasilianum and Plasmodium simium in South America, and Plasmodium vivax-like parasites from apes in West Africa.
The clinical significance of Plasmodium knowlesi malaria is increasingly recognized in Southeast Asia after the initial discovery of a focus of human infections in the Kapit division of the Sarawak state in Malaysian Borneo, subsequently in peninsular Malaysia and then in other Southeast Asian countries. Plasmodium knowlesi is normally a parasite of Macaca fascicularis (the long-tailed or crab-eating macaque), M. nemestrina (the pig-tailed macaque), Trachypithecus obscuras (dusky leaf monkey or spectacled langur) and Presbytis melalophus (banded leaf monkey or brown langur).
Plasmodium knowlesi infections in humans are commonly misidentified as P. malariae and P. falciparum during routine microscopic examination of Giemsa stained blood smears because of morphological similarities between the blood stages. Therefore, the definitive identification of human infections caused by P. knowlesi presently relies on PCR-based techniques. The diagnosis of P. knowlesi malaria is important in a clinical context because its pathogenicity and drug treatment options can differ from the human malaria caused by P. falciparum, P. malariae, P. ovale and P. vivax that are also prevalent in the Southeast Asian region. Rapid immunochromatography-based diagnostic tests (RDTs) are being increasingly used worldwide to detect malaria but a reliable RDT for specifically detecting P. knowlesi is not yet available. PCR-based assays are sensitive enough to detect 1-5 parasites per μl of blood and able to discriminate between the different Plasmodium species that infect humans, including P. knowlesi. However PCR-based assays are relatively expensive, time consuming, and require specialized equipment. PCR-based techniques are therefore not suitable for routine diagnosis in typical peripheral and district-level diagnostic laboratories, but valuable for use in central reference laboratories in the P. knowlesi-endemic regions of Southeast Asia.
What is needed are reagents and methods for the rapid and accurate detection and discrimination of malaria causing Plasmodium knowlesi. 